Positioning entities is done by specifying the offset on the x, y and z axis.

  position="0 2 -2"

If an entity is inside another entity, the positioning is relative to the parent entity.

<a-entity position="-2 2 -2">
  <a-box position="-2 2 -2">
    <!-- This box will actually be at -4 4 -4 relative to the scene-->

Further reading: